Tyrannosaurus rex was one of the most fearsome creatures to walk the planet. Some 65 million years ago, this 12-meter (40-foot) long meat-eating dinosaur chomped its prey using teeth the size of bananas. Yet scientists have wondered why its ancestors remained relatively small for millions of years. Now, the discovery of a new meat eater may offer some answers. This newfound predator may have preyed on T. rex’s ancestors. That should have kept them small and on the run.